Infrared devices can be either infrared emitters, or infrared detectors. Micro-machined IR emitters and detectors have the advantage of low cost and small size. However, packaging can often greatly increase the size and cost of the devices, and hence partially negate these advantages. In particular the use of the filter, window or lens can greatly increase cost. Furthermore, for thermal IR emitters or detectors it is generally beneficial to have the device packaged in vacuum or a gas with low thermal conductivity as it greatly reduces the power consumption for IR emitters, and increases the sensitivity of IR detectors. However, vacuum packaging or gas sealing at a package level can be very expensive and for some packaging technologies is not possible.
Thermal infrared emitters and infrared detectors are both well known in literature and the mininituarised versions are fabricated using micro-machined processes. Thermal infrared emitters typically include a resistive micro-heater embedded within a thin membrane and supported on a silicon substrate. When current is passed through the heater, it heats up to a high temperature (which can be as much as 700° C. or even higher), and at this high temperature, the device emits infrared radiation.
A number of designs of IR emitters have been reported.
For example, Parameswaran et. al. “Micro-machined thermal emitter from a commercial CMOS process,” IEEE EDL 1991 reports a polysilicon heater for IR applications made in CMOS technology, with a front side etch to suspend the heater and hence reduce power consumption. Similarly, D. Bauer et. Al. “Design and fabrication of a thermal infrared emitter” Sens & Act A 1996, also describes an IR source using a suspended polysilicon heater. U.S. Pat. No. 5,285,131 by Muller et al.; US2008/0272389 by Rogne et. Al; and San et. al. “A silicon micromachined infrared emitter based on SOI wafer” (Proc of SPIE 2007) also describe similar devices using a polysilicon heater.
Yuasa et. al “Single Crystal Silicon Micromachined Pulsed Infrared Light Source” Transducers 1997, describe an infrared emitter using a suspended boron doped single crystal silicon heater. Watanabe, in EP2056337, describes a suspended silicon filament as an IR source. The device is vacuum sealed by bonding a second substrate. Cole et. al. “Monolithic Two-Dimensional Arrays of Micromachined Microstructures for Infrared Applications” (proc of IEEE 1998) describe an IR source on top of CMOS processed device.
Designs based on a platinum heater have also been described. For example, Hildenbrand et. al. “Micromachined Mid-Infrared Emitter for Fast Transient Temperature Operation for Optical Gas Sensing Systems”, IEEE Sensor 2008 Conference, reports on a platinum heater on a suspended membrane for IR applications.
Similarly Ji et. Al. “A MEMS IR Thermal Source For NDIR Gas Sensors” (IEEE 2006) and Barritault et. al “Mid-IR source based on a free-standing microhotplate for autonomous CO2 sensing in indoor applications” (Sensors & Actuators A 2011), Weber et. al. “Improved design for fast modulating IR sources”, Spannhake et. Al. “High-temperature MEMS Heater Platforms: Long-term Performance of Metal and Semiconductor Heater Materials” (Sensors 2006) also describe platinum based as well as other emitters.
Some other IR emitter designs are disclosed by U.S. Pat. No. 6,297,511 by Syllaios et. al., U.S. Pat. Nos. 5,500,569, 5,644,676, 5,827,438 by Bloomberg et. al, and WO 02/080620 A1 by Pollien et. al.
Thermal IR detectors on a silicon substrate comprise a thin membrane layer (made of electrically insulating layers) that is formed by etching of part of the substrate. Heating due to incident IR radiation increases the temperature of the membrane—which can be measured by either a thermopile, a resistor, or a diode.
For Example, Schneeberger et. al “Optimized CMOS Infrared Detector Microsystems,” Proc IEEE Tencon 1995, reports fabrication of CMOS IR detectors based on thermopiles. The thermopile consists of several thermocouples connected in series. KOH (potassium hydroxide) is used to etch the membrane and improve the thermal isolation. Each thermocouple includes 2 strips of different materials, connected electrically and forming a thermal junction at one end (termed hot junction) while the other ends of the material are electrically connected to other thermocouples in series forming a thermal cold junction. The hot junctions of the thermocouples are on the membrane, while the cold junctions are outside the membrane. Three different designs of the thermocouples are given in the paper with different material compositions: either Aluminium and p-doped polysilicon, Aluminium and n-doped Polysilicon, or p-doped polysilicon and n-doped polysilicon. Heating due to absorption of incident IR radiation causes a slight increase in the temperature of the membrane. The Seebeck effect causes a slight voltage difference across each thermocouple—resulting in a much larger increase in voltage difference across the thermopile which is the sum of the voltages across each thermocouple.
Several other thermopile devices are described by Graf et. al. “Review of micromachined thermopiles for infrared detection” Meas. Sci. Technol. 2007.
Another method of detecting IR radiation is by the use of thermodiodes. For example, Kim “A new uncooled thermal infrared detector using silicon diode,” S&A A 89, 2001, describes a diode fabricated by micromachining for use as an IR detector. However, the dome shaped silicon nitride window can be fragile and the irregular shape can affect the emission profile of the device.
However the packaging in all these uses metal, ceramic or plastic packaging. These can be TO packages, as in: San et. Al. “A silicon micromachined infrared emitter based on silicon on insulator (SOI) wafer,” SPIE Digital Library 2008; Hildenbrand et. Al. “Micromachined Mid-Infrared Emitter for Fast Transient Temperature Operation for Optical Gas Sensing Systems” Proceedings of IEEE Sensors Conference 2008; Ji et. Al. “A MEMS IR Thermal Source for NDIR Gas Sensors” (IEEE 2006).
Several SMD (surface mount device) packages are also found in commercial IR products.
There are however very few reports of chip or wafer level packaged sensors.
U.S. Pat. No. 5,285,131 describes an IR emitter, consisting of a polysilicon filament suspended over a silicon substrate and sealed in a vacuum with a dome shaped silicon nitride window. However, the filament is supported on only 2 sides, and so is relatively fragile. Furthermore, as it is a filament, the heated area is very small, resulting is a lower amount of IR radiation. Lastly, the dome shaped silicon nitride window is not very easy to fabricate, particularly to ensure that it does not buckle.
US20050081905 describes a thermopile IR detector encapsulated by sealing on the top and bottom at chip level. However, the cavity in the package in not a vacuum, or a low thermal conductivity gas, so the performance of the sensor is not enhanced. The package simply provides a small low cost packaged device.
U.S. Pat. No. 7,741,625 discloses an IR emitter packaged at chip level in vacuum. However, the device uses a silicon membrane which greatly increases power consumption. The device has no method to maintain the vacuum in the cavity which may slowly increase in pressure over time due to minor leakages. It also relies on providing electrical connection to the emitter through the semiconductor substrate which typically has high resistance resulting in high voltage requirements and/or higher power consumption.
U.S. Pat. No. 6,036,872 and CN102583220 generally relate to wafer level and vacuum packaging.